China's Lenovo Group Ltd, the world's largest PC maker, had pre-installed a virus-like software on laptops that makes the devices more vulnerable to hacking, cybersecurity experts said on Thursday.

Users reported as early as last June that a program called Superfish pre-installed by Lenovo on consumer laptops was 'adware', or software that automatically displays adverts.

Robert Graham, CEO of U.S.-based security research firm Errata Security, said Superfish was malicious software that hijacks and throws open encrypted connections, paving the way for hackers to also commandeer these connections and eavesdrop, in what is known as a man-in-the-middle attack.

An administrator on Lenovo's official web forum said on Jan. 23 that Superfish has been temporarily removed from consumer computers. Lenovo executives were not immediately available for comment during the Lunar New Year holiday in China.

Graham and other experts said Lenovo was negligent, and that computers could still be vulnerable even after uninstalling Superfish. The software throws open encryptions by giving itself authority to take over connections and declare them as trusted and secure, even when they are not.

"The way the Superfish functionality appears to work means that they must be intercepting traffic in order to insert the ads," said Eric Rand, a researcher at Brown Hat Security. "This amounts to a wiretap."